Low-carbon strip steels 39in the structure and may form iron carbides. The second is the newer type of
steel called interstitial-free (IF) steel in which a strong carbide-forming element
is present, such as titanium or niobium, to combine with some or all of the
carbon usually to leave very little carbon in solid solution. The IF steels usually
have an ultra-low-carbon content well below 0.005%, whereas the AK steels
usually have a carbon content above 0.015%. The general chemical limits are
given in Table 1.3. The more usual IF steels and the AK steels when batch
annealed are almost completely non-ageing, whereas an AK steel may exhibit
varying degrees of ageing after continuous annealing, depending on the chemistry
and annealing conditions used. Some batch-annealed AK steels, however, will
exhibit a small amount of room temperature strain ageing if they are intended to
be bake-hardening steels.
An IF steel is the only type of steel that can satisfy the formability require-
ments of the Fe P06 grade, whereas a batch-annealed AK steel may be used to
satisfy the formability requirements of the Fe P05 grade. A continuously annealed
AK steel would only be expected to satisfy the formability requirements of this
grade if very specific chemistry, hot rolling conditions and continuous annealing
cycles are used. It would be more usual to use an AK steel only for the Fe P03
and P01 grades. It would also be more usual to use an IF steel to satisfy the
requirements of the Fe P05 grade, if continuous annealing is the only method
that is available.
Clearly, it is possible to use the chemistry and prior processing necessary for
a higher grade to satisfy the requirements of a lower grade. This may be done
either to reduce the number of independent steel chemistries that are needed, to
be able to reduce the complexity of the annealing cycle, or to be sure that the
property requirements would be easily met. Further information on the effects
of chemistry and processing on the properties of mild steel are given in the
following sections.
Low-strength, interstitial-free (IF) steels
Early work 62'63 showed that very low-carbon steels could develop high rm and
elongation values when sufficient titanium was added to tie up all the carbon and
nitrogen and that the rm value increased with increasing titanium content. Later
work showed that it was also possible to obtain good r values by the addition
of niobium, ~ and steels alloyed with titanium and niobium either separately or
together are now in common use. Recent work has shown, however, that there
may be some potential for IF steels containing vanadium. 65
From the early 1970s, carbon contents close to 0.01% became readily available
and it was then necessary to add sufficient alloying addition to be able to combine
with all the carbon and nitrogen in the steel and to leave a small surplus in order
that very good r values were obtained. This effect is illustrated in Figure 1.37(a)
and (b) for both titanium and niobium additions.
More recently, with improvements in vacuum degassing techniques, ultra-low-
carbon (ULC) contents below 0.003% have become easily available and with
these ultra-low-carbon contents, it has been possible to lower the alloy addition
while still achieving high mean r values. The original sharp rise in r value,